DCOM Viva Questions FOR THIRD YEAR STUDENTS.pdf
- 1. 1. What are the disadvantages of Analog communication? Its not reliable, Noise effect is more on the signals, Power required for signal transmission also more, Circuit complexity is more and costly. 2. What are the Advantages of Digital Communication? Reliable, Noise effect is very less, power consumption is very less, various Digital ICs are available so circuits not complex , cheap, Error detection and correction is also possible. 3. What are different types of digital modulation? ASK, FSK, PSK, PCM, DPCM,Delta modulation, Adaptive Delta modulation etc.. 4. How to convert an analog signal into digital signal? Blocks: Anti aliasing filter, Sampler, Quantizer, encoder. 5. Define the functionality of Sampler, Quantizer? Sampler: converts a continuous time signal into discrete time signal. Quantizer: converts continuous in amplitude signal into discrete in amplitude signal. 6. What are some Coding techniques? Pulse code modulation, Differential pulse code modulation, Delta modulation, Adaptive delta modulation. 7. What is Sampling? Converting a continuous time signal into discrete in time signal is called as Sampling (similar to cutting a bread into slices) 8. Define Sampling theorem? To reconstruct the Continuous time signal from discrete time signal ,the sampling frequency should be more than equal to twice of Continuous time signal frequency(max). 9. What is Nyquist Rate? If the sampling frequency is twice of Continuous time signal frequency(max), then that is called as Nyquist rate. 10. How many types of samplings are their? Explain briefly? Impulse Sampling, Natural sampling, Flat top sampling. 11. What is aliasing effect? How to overcome it? Due to imperfect sampling the signals will be interfered in frequency domain i.e called aliasing effect in sampling. if sampling theorem satisfied in sampling or first by passing signal from anti aliasing filter before sampling then aliasing effect will be reduced 12. What are the Analog pulse modulation methods? Pulse amplitude modulation, pulse width modulation and pulse position modulation.. 13. Define Pulse amplitude modulation? The carrier pulse height (amplitude) proportional to amplitude of messege signal.
- 2. 14. Define Pulse width modulation? The carrier pulse width proportional to amplitude of message signal. 15. Define Pulse position modulation? The carrier pulse position proportional to amplitude of message signal. 16. Compare PAM, PWM, PPM? 17. What is Amplitude shift Keying (ASK) ? It represent the digital data as variations in amplitudes in carrier wave. i.e '1' represented by transmitting a fixed amplitude carrier wave for the bit duration with constant frequency. 18. What is Phase shift keying ( PSK)? It represent the digital data as variations in phase shift in carrier wave. i.e '1' represented by 0 phase shift carrier wave , where '0' represented as 180 phase shift incarrier wave for the bit duration with constant frequency 19. What is Frequency shift keying (FSK) ? It represent the digital data as variation in frequency in carrier wave, i.e for '1' more than carrier frequency , for '0' less than carrier frequency. 20. What is Binary Phase shift Keying (BPSK) ? for each one bit of binary data (0 & 1) carrier phase will be changed (two different shifts: 0, 180) 21. What is Quadrature Phase shift Keying (QPSK) ? for each two bits of binary data (00,01,10 & 11) carrier phase will be changed (four different shifts : 45, 135, -45, -135) 22. What is the difference between Bit Rate and Baud Rate? Bit rate represents Bits per sec,Baud rate represents no. of symbols per second i.e. in communications the no. of bits transmitted per sec is called as Bit Rate (units bps) and The no. of times a signal (here carrier) changes its state (change in freq, phase, amplitude) per sec is called as Baud rate. 23. What is bandwidth of BPSK signal? 2Fc, if Fc represents carrier frequency 24. Compare ASK, PSK and FSK.? Bandwidth: ASK< PSK < FSK Power: ASK <PSK = FSK Probability of error: ASK > PSK > FSK Signal to Noise Ratio: ASK < PSK < FSK 25. Why is ASK called as ON-OFF keying? When input data is 1 then output is carrier, if input is 0 out put is zero. so its looks like a switch which will switch on when input is 1 and off when input is zero
- 3. 26. Define Pulse code modulation? Each and every quantized samples will be encoded with sequence of zeros and ones with 'n' bits within sampling interval (Ts), So the bit duration will be Ts/n. as no. of bits (n) increases error decreases but bandwidth increases. 27. How bits are needed to encode N different levels? Log2 N 28. Define step size? It is the difference between two successive no.s (levels), i.e. voltage difference between one digital level and the next digital level. (Ex: 000 & 001, 1011 & 1100) 29. How to calculate Step size in PCM? step size(Δ) = (VFS-Vmin)/L where L =2n , n = no. of bits, VFS = full scale voltage 30. Define Quantization error? It is the difference between sampled signal and Quantized signal. 31. What is the max value of Quantization error? Half of the Step size. 32. What are the applications of PCM? computer Disk, Digital Telephony, Digital Audio Applications, etc. 33. What are the disadvantages of Pulse code modulation? To get less Quantization Noise no. of bits should be increased, then bandwidth will be increased. So we have to compromise with either Quantization error or bandwidth. 34. Define Differential pulse code modulation? In DPCM, The difference between the consequent samples will be encoded with sequence of zeros and ones. 35. Why DPCM is better than PCM? Instead of encoding each sample, Its better to encode the difference between samples then Quantization error will be minimized with less no. of bits, then bandwidth also get decreased. 36. Define Delta modulation? Why it is better? It is same as DPCM with no. of bits to encode is one bit only (either 0 or 1). by this bandwidth will be decreased. 37. What is granular noise? Define slope overload? The Delta modulation is efficient when and only when signal is varying continuously with less variations. if signal varies suddenly then we get two different Noises. those are slope overload and granular noise 38. When granular noise and slope overload occur in Delta modulation?
- 4. Granular Noise: Δ / Ts > slope of signal Slope Overload Noise: Δ / Ts < slope of signal 39. What is Adaptive Delta Modulation and what are the advantages? If the step size varies according to the slope of the signal then that is called as Adaptive Delta modulation. granular and slope over load noise will be desuced. 40. Compare all Digital pulse modulation techniques (PCM, DPCM, DM, ADM)? 41. What is multiplexing? How many types of multiplexing possible in communication? Combining two or more signals to pass through a channel is called as multiplexing. The different Multiplexing techniques are:1) Frequency division Multiplexing, 2) Time division Multiplexing, 3) Wavelength division multiplexing, 4) Orthogonal frequency division multiplexing. 42. What is Line coding and what are the different line coding techniques? Line coding is the representation of digital data (0,1) as puses. the types in line coding are: Non Return to Zero (NRZ), Return to Zero (RZ), Biphase, Differential Manchester, Bipolar 43. What is the difference between Source coding and Line coding? Source Coding is used to represent analog signals in form of Digital data (like PCM,DPCM,DM), where Line Coding is used to represent digital data in the form of pulses (like NRZ, RZ) 44. Define ISI (Inter symbol Interference)? It is a Distortion in digital signal that one symbol interferes with other symbol. 45. What is Matched filter? It is an optimal linear filter for maximizing the SNR (Signal to Noise Ratio) in the presence of additive random noise. 46. What is the cause of inter symbol Interference (ISI) and it can be reduced?
- 5. ISI caused by multipath propagation and inherent non linear frequency response of channel. It can be reduced by pulse shaping. 47.What is multipath Interference? When a signal reaches to transmitter in various paths then delays exist in each path so at receiver all delayed signals will be received, then original signal will be interfered by its delayed versions. That is called multipath interference. 51. What is frequency synthesizer? it is an oscillator which can generate any range of frequencies. Applications of frequency synthesizer? 52. Define polar encoding? It is a line code of RZ (Return to Zero) in which binary 1 represented by +ve voltage and 0 represented by zero volts. 53. Define bipolar encoding? It is a line code of NRZ (Non Return to Zero) in which two non zero values are for encoding binary data. 54. Define Manchester encoding? It is a line code which will generated based on clock and binary data. The code is XOR logic of clock and digital data. 55. What are the features of Code Division Multiple Access? The CDMA technique is more secure communication over all, this is used in military, If we know the code which is used to encode the data, then only we can receive the data. But No. of users are limited (depends on code length) 56. Explain about Frequency Division Multiple Access? In FDMA Spectrum is divided into segments and each segment is permanently assigned to each transmitter. so to get more efficiency in using spectrum, each signal bandwidth should be less and perfect modulation technique should be used. 57. Explain about Time division Multiple Access? In this a fraction of time slot will be assigned to each transmitter, so as no. of transmitters increases complexity increases. for this perfect synchronization is required.
- 6. What is the significance of the constellation diagram in digital modulation? Answer: A constellation diagram visually represents the modulation scheme of a digital signal, showing the possible states (symbols) that the signal can take. It helps in analyzing performance, detecting errors, and understanding the impact of noise. What are the common sources of errors in digital communication? Answer: Common sources of errors include thermal noise, interference from other signals, multipath fading, and signal distortion due to bandwidth limitations or inadequate signal strength. Explain the concept of error detection and correction. Answer: Error detection involves identifying whether errors have occurred during data transmission (using techniques like checksums or cyclic redundancy checks), while error correction refers to the process of identifying and correcting those errors (using methods like Hamming codes or Reed- Solomon codes). What is the difference between forward error correction (FEC) and automatic repeat request (ARQ)? Answer: FEC allows the receiver to detect and correct errors without needing retransmission, while ARQ requires the sender to retransmit the data upon detection of an error. What is Shannon’s theorem? Answer: Shannon's theorem states that the maximum data rate (channel capacity) that can be transmitted over a noisy channel is determined by the bandwidth and the signal-to-noise ratio (SNR) of the channel. The formula is C=Blog2(1+SNR)C = B log_2(1 + text{SNR})C=Blog2 (1+SNR). Define channel capacity. How is it calculated? Answer: Channel capacity is the maximum rate at which information can be transmitted over a communication channel without error. It is calculated using Shannon’s theorem and is dependent on the bandwidth of the channel and the noise characteristics. What is the significance of the Nyquist rate? Answer: The Nyquist rate is the minimum sampling rate required to accurately reconstruct a signal without aliasing. It is twice the highest frequency present in the signal, ensuring that no information is lost during sampling.
- 7. What is pulse code modulation (PCM)? Answer: PCM is a method of digitally representing analog signals. In PCM, the analog signal is sampled at uniform intervals, and each sample is quantized to the nearest value within a range, resulting in a series of binary values. What is spread spectrum technology? Give examples. Answer: Spread spectrum technology spreads the signal over a wide frequency band, making it more resistant to interference and eavesdropping. Examples include Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS) What are some applications of digital communication in modern technology? Answer: Applications include mobile communication, satellite communication, digital broadcasting, data transmission over the internet, and wireless sensor networks, among others. Discuss the impact of 5G technology on digital communication. Answer: 5G technology significantly enhances digital communication by providing higher data rates, lower latency, increased capacity, and support for a greater number of connected devices. It enables advancements in IoT, smart cities, and real-time applications. What are the challenges faced in digital communication systems today? Answer: Challenges include dealing with increasing data traffic, ensuring cybersecurity, maintaining signal quality in wireless environments, managing interference, and addressing the digital divide in access to technology. How do you perform bit error rate (BER) testing? Answer: BER testing involves transmitting a known sequence of bits through a communication system and comparing the received bits with the original. The BER is calculated as the ratio of the number of erroneous bits to the total number of transmitted bits. What tools are used for digital communication simulations? Answer: Common tools include MATLAB, Simulink, GNU Radio, and software-defined radio (SDR) platforms, which allow for modeling and simulating various communication systems and algorithms.
- 8. What is a random process? Answer: A random process, or stochastic process, is a collection of random variables indexed by time or space. It describes systems that evolve over time in a way that is inherently unpredictable, such as the fluctuation of signal levels in communication systems. Explain the difference between stationary and non-stationary processes. Answer: A stationary process has statistical properties (like mean and variance) that do not change over time. In contrast, a non-stationary process has statistical properties that vary with time, making it more complex to analyze. What is the significance of the autocorrelation function in random processes? Answer: The autocorrelation function measures the similarity between values of a random process at different times. It helps analyze the predictability and periodicity of the signal, and is crucial in understanding how noise affects signal transmission. Define white noise. What are its characteristics? Answer: White noise is a random signal with a constant power spectral density across all frequencies. Its characteristics include a flat frequency response and the absence of correlation between different samples (uncorrelated random variables). What are the main types of noise in communication systems? Answer: The main types of noise include: 1. Thermal noise: Caused by the random motion of electrons in conductors. 2. Shot noise: Arises from the discrete nature of charge carriers (like electrons) in a conductor. 3. White noise: Has equal intensity at different frequencies. 4. Flicker noise: Also known as 1/f noise, it has a power density inversely proportional to frequency. How does noise affect digital communication? Answer: Noise introduces errors in the transmitted data by altering the received signal. It can cause bit errors, degrade signal quality, and reduce the overall performance of communication systems.
- 9. What is the Signal-to-Noise Ratio (SNR), and why is it important? Answer: The Signal-to-Noise Ratio (SNR) is the ratio of the power of the desired signal to the power of the background noise. It is important because a higher SNR indicates a clearer signal, leading to lower error rates and better communication quality. Explain the concept of the noise figure. Answer: The noise figure (NF) quantifies how much a device (like an amplifier) degrades the SNR of a signal. It is defined as the ratio of the input SNR to the output SNR, expressed in decibels (dB). A lower NF indicates better performance. What is the role of random processes in the analysis of communication systems? Answer: Random processes are essential for modeling the behavior of signals and noise in communication systems. They help in the analysis of system performance, design of modulation schemes, and evaluation of error rates. How is the Central Limit Theorem applicable in digital communication? Answer: The Central Limit Theorem states that the sum of a large number of independent random variables tends to be normally distributed, regardless of their individual distributions. In digital communication, this is useful for approximating noise and signal distributions in large systems, leading to simpler analysis. What is a Gaussian process? Answer: A Gaussian process is a type of random process where any finite collection of random variables follows a multivariate Gaussian distribution. In communication, Gaussian noise is often assumed because it simplifies analysis and modeling. How can you mitigate the effects of noise in communication systems? Answer: Noise can be mitigated using techniques such as: 1. Error correction codes: Detect and correct errors in transmitted data. 2. Filtering: Remove unwanted noise from the signal. 3. Modulation techniques: Choose robust modulation schemes (like QAM or PSK) that are less sensitive to noise. 4. Diversity techniques: Use multiple channels or paths to improve reliability.